Network analyzers are instruments that measure transfer and/or impedance functions of networks. A sine wave signal source stimulates the device under test. Since transfer and impedance functions are ratios of various voltages and currents, a means for separating the signals of interest from the measurement ports of the device under test is required. The network analyzer detects the separated signals, forms the desired signal ratios and displays the results.
Measurements of voltages and currents are difficult at high frequencies. High frequency network behavior is best described using transmission line theory. Scattering parameters, or S-parameters, were developed to characterize networks at high frequencies. S-parameters define the ratios of reflected and transmitted traveling waves measured at the network ports. In order to measure the S-parameters, it is necessary to separate the transmitted or incident signal and the signal from the device under test. A device known as a test set is utilized to interconnect the test signal source, the device under test and the network analyzer. For example, in the case of reflection measurements, the test set supplies a test signal to the device under test and separates incident and reflected signals for measurement and analysis by the network analyzer. The test set provides a representation of the incident signal on a reference port and a representation of the reflected signal on a coupled port.
Prior art S-parameter test sets have utilized a directional bridge for separating the incident signal and the reflected signal. In one prior art test set, the Model HP 85046A/B S-Parameter Test Set manufactured by Hewlett-Packard Company, the incident signal is provided to the reference port through a power splitter, and a symmetric wheatstone bridge is used for separating incident and reflected signals and providing the reflected signal to the coupled port. While this device provides generally satisfactory performance, it has certain limitations. The power splitter and the directional bridge do not track over a wide range of frequencies because of their different structures. The bridge and the power splitter each have 6 dB loss, resulting in 12 dB loss to the test port. As a result, the power available to the device under test is limited. Other limitations include a source mismatch due to a long RF cable interconnecting the power splitter and the bridge, and degraded performance at the upper and lower ends of the frequency range.
Another prior art S-parameter test set, the Model HP 85047A S-Parameter Test Set manufactured by Hewlett-Packard Company, utilizes a first directional bridge for monitoring the incident signal at the reference port and a second directional bridge for monitoring the reflected signal. The bridges are matched, thereby providing improved tracking. In addition, each bridge is asymmetric and has only 1.5 dB loss for a total loss of 3 dB to the test port. Each bridge is packaged in a separate RF housing. While the test set utilizing two bridges has improved performance in comparison with the test set utilizing a power splitter, it has several limitations. The two bridges are relatively expensive, since two precision-milled housings are required. In addition, the two bridge assemblies have a relatively large volume and are not well suited for compact products. Separation of the bridges by a long RF cable causes a poor match due to standing wave ratio interactions. Since the two bridges are in different packages, temperature gradients can cause them to change coupling at different rates. Finally, performance is degraded at low frequencies.
It is a general object of the present invention to provide an improved S parameter test set.
It is another object of the present invention to provide an S-parameter test set capable of operating over a wide frequency range.
It is a further object of the present invention to provide an S-parameter test set which is small in volume and low in cost.
It is a further object of the present invention to provide an S-parameter test set having close tracking between a reference channel and a coupled channel over a wide frequency range.
It is yet another object of the present invention to provide an S-parameter test set wherein the source and the device under test are well matched over a wide frequency range.